The present invention relates generally to inductive sensors, and more particularly to a method for detecting a change in the inductance of an inductive sensor.
Conventional inductive sensors may use an inductive coil positioned relative to a magnetostrictive object such that magnetic flux lines induced by an alternating electric current in the coil pass through the object in a direction substantially parallel to the strain direction. The inductance of the coil is measured over time. A change in permeability of the object due to a change in strain of the object is detected or determined from a change in the measured inductance over time.
What is needed is an improved method for accurately detecting a change in the inductance of such inductive sensors as well as other variable inductance elements.
In a first aspect, a method for detecting changes in inductance of a variable inductance element involves the steps of: a) producing an oscillating signal having a frequency that varies in proportion to variations in inductance of the variable inductance element; b) producing an intermediate analog voltage that varies in proportion to variations in frequency of the oscillating signal of step a); c) scaling the intermediate analog voltage of step b) to produce an output analog voltage; and d) detecting changes in inductance of the variable inductance element based upon changes in the output analog voltage of step c).
In another aspect, a method is provided to convert a known range of inductance change of a variable inductance element between a first inductance and a second inductance into a desired range of analog voltage change between a first voltage level and a second voltage level. The method involves the steps of: a) establishing an oscillator circuit incorporating the variable inductance element so as to produce an oscillating signal having a frequency that varies with inductance of the variable inductance element, the oscillating signal produced with a first frequency when the variable inductance element has the first inductance and produced with a second frequency when the variable inductance element has the second inductance; b) establishing a circuit to convert the frequency of the oscillating signal to an intermediate analog voltage, the intermediate analog voltage produced at a first intermediate level when the oscillating signal has the first frequency and produced at a second intermediate level when the oscillating signal has the second frequency; and c) establishing a circuit to scale the intermediate analog voltage so as to produce an output voltage within the desired range, the output voltage produced at the first voltage level when the intermediate analog voltage is at the first intermediate level and produced at the second voltage level when the intermediate analog voltage is at the second intermediate level.
In a further aspect, a circuit for producing a voltage level substantially proportional to inductance of a variable inductance element includes an oscillator stage having the variable inductance element connected therein and producing an oscillating signal having a frequency that varies with inductance of the variable inductance element. A conversion stage is operatively connected to receive the oscillating signal and produces an intermediate analog voltage that varies in proportion to variations in the frequency of the oscillating signal. An amplification stage is operatively connected to receive the intermediate analog voltage and operates to offset and amplify the analog voltage to produce an output analog voltage with a voltage level proportional to inductance of the variable inductance element.
The foregoing methods and circuit provide a practical, effective and relatively inexpensive way to detect changes in inductance of a variable inductive element.